Reversible cross-tolerance to platelet-activating factor signaling by bacterial toxins

Abstract

Bacterial toxins signaling through Toll-like receptors (TLRs) are implicated in the pathogenesis of many inflammatory diseases. Among the toxins, lipopolysaccharide (LPS) exerts its action via TLR-4 while lipoteichoic acid (LTA) and bacterial lipoproteins such as Braun lipoprotein (BLP) or its synthetic analogue Pam3CSK4 act through TLR-2. Part of the TLR mediated pathogenicity is believed to stem from endogenously biosynthesized platelet-activating factor (PAF)- a potent inflammatory phospholipid acting through PAF-receptor (PAF-R). However, the role of PAF in inflammatory diseases like endotoxemia is controversial. In order to test the direct contribution of PAF in TLR-mediated pathogenicity, we intraperitoneally injected PAF to Wistar albino mice in the presence or absence of bacterial toxins. Intraperitoneal injection of PAF (5 μg/mouse) causes sudden death of mice, that can be delayed by simultaneously or pre-treating the animals with high doses of bacterial toxins- a phenomenon known as endotoxin cross-tolerance. The bacterial toxins- induced tolerance to PAF can be reversed by increasing the concentration of PAF suggesting the reversibility of cross-tolerance. We did similar experiments using human platelets that express both canonical PAF-R and TLRs. Although bacterial toxins did not induce human platelet aggregation, they inhibited PAF-induced platelet aggregation in a reversible manner. Using rabbit platelets that are ultrasensitive to PAF, we found bacterial toxins (LPS and LTA) and Pam3CSK4 causing rabbit platelet aggregation via PAF-R dependent way. The physical interaction of PAF-R and bacterial toxins is also demonstrated in a human epidermal cell line having stable PAF-R expression. Thus, we suggest the possibility of direct physical interaction of bacterial toxins with PAF-R leading to cross-tolerance.

Keywords:

• Braun lipoprotein
• cross-tolerance
• lipopolysaccharide
• lipoteichoic acid
• platelet-activating factor

Acknowledgements

The authors thank the National Fellowship for Higher Education for Schedule Tribes, Basic Science Research. The Department of Studies in Biochemistry, University of Mysore, received financial assistance from the Special Assistance Programme and the Vision Group of Science and Technology, Government of Karnataka, India. This study was in-part supported by Elsa U. Pardee Foundation grant [670645]. The authors thank Jeffrey B Travers (Wright State University, Dayton, USA) for providing stably PAF-R expressing KBP-cell line.

Disclosure Statement

The authors declare no conflict of interest

Author Contributions

GKM envisioned the study and thoroughly edited the manuscript. KVA performed the major experiments while MSS and AT performed minor experiments. RPS and KK read and evaluated experimental data. KVA and GKM wrote the paper.

Supplementary Material

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